1. Field of the Invention
The present invention relates to a light measuring circuit which is applicable to an automatic flash unit or electronic shutter of a camera.
2. Description of the Prior Art
Generally, in a light measuring circuit for use in an automatic flash unit, an electronic shutter of cameras and so forth, the light measuring sensitivity must be changed in accordance with an exposure condition which is provided by a combination of coefficients concerning exposure such as the film sensitivity, the F-number of a camera lens, etc.; heretofore, a variety of light measuring sensitivity switching systems have been proposed. For example, Japanese Utility Model Application Gazette No. 13953/79 sets forth such a system in which a plurality of integrating capacitors of different capacitances are selectively changed over to alter the charging time constant of a light integrating circuit, whereby to switch the light measuring sensitivity. This system is highly stable in operation independently of a temperature change, high in response speed and capable of responding to a wide variety of exposure conditions as compared with other systems, for example, of the type that the light measuring sensitivity is switched by changing a voltage to be applied to a photodetector or by changing a reference voltage for detecting an integrated voltage level; therefore, the abovesaid system is now widely put to practical use. In this system, however, since a plurality of integrating capacitors having discrete capacitance values are changed over, the light measuring sensitivity can be switched only in a stairstep manner and this poses the following problems:
Namely, films now placed on the market have ten-odd kinds of sensitivities, mainly ranging from ASA Speed 25 to ASA Speed 800, and they are mostly divided into a series of even multiples of 25 and a series of even multiples of 32. The practical range of the F-number of camera lenses is from 1.4 to 22; the F-number is indicated in the form of F1.4, F2.0, F2.8, . . . by a geometric progression using .sqroot.2 as a common ratio and a click stop is usually provided midway between adjacent values to subdivide the F-number. Since the values of the F number of the camera lens and the film sensitivity are thus diversified, the exposure conditions which are determined by combinations of them are very large in number. If the light measuring circuit of the automatic flash unit cannot set the sensitivity in response to all of such a great number of exposure conditions, then photographing conditions are limited or correct exposure cannot be obtained.
However, for switching the sensitivity by the aforesaid system of the type selectively changing over integrating capacitors in response to variations of the exposure conditions, integrating capacitors are required in huge numbers; this is impractical and almost impossible. Therefore, in the prior art about five sensitivity set values at most are determined so that the light measuring sensitivity may fit only for the film sensitivity most widely used, in many cases, the even-multiple series of ASA Speed 25. Accordingly, there is the problem such that under exposure conditions other than specified ones, the light measuring sensitivity cannot be adjusted to a proper light control value. Such a problem can be solved by the system of changing the voltage to be applied to the photocell or changing the reference voltage for the detection of an integration voltage level, but these systems have the drawback that they cannot respond to a wide variety of exposure conditions, resulting in the photographing conditions being limited, as referred to previously.